JP3789362B2 - Switching power supply - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply device having a circuit that prevents an overshoot of an output voltage at the time of startup.
[0002]
[Prior art]
A so-called two-stage converter that combines a step-down converter at the front stage and a half-bridge converter at the rear stage has been proposed as one of switching power supply devices that are used in electronic systems such as computers and that supply an efficient and stable voltage. ing.
[0003]
The step-down converter steps down the input voltage to a certain voltage level. The half bridge converter rectifies and smoothes the voltage generated in the secondary winding by exciting the primary winding of the transformer by a half bridge circuit in which the first and second switching elements are turned on and off at a constant duty. Generate DC voltage.
[0004]
A switching power supply device having such a configuration has a so-called soft start circuit that prevents noise generation, current to circuit elements, voltage stress, and the like and smoothly outputs an output voltage when the power supply is started. is there.
[0005]
The feedback circuit includes an error amplifier and a photocoupler. During normal operation, the error amplifier compares the reference voltage with the divided voltage obtained by dividing the output voltage to control the photocoupler current. If the output voltage is considerably lower than the set voltage when the power is turned on, The photocoupler is OFF.
[0006]
During this period when the photocoupler is OFF, the PWM drive circuit that drives the switch element of the step-down converter generates a PWM waveform based on the soft start signal.
[0007]
This soft start signal is a voltage generated by charging a capacitor with a constant current. OUT of the drive control circuit outputs a pulse whose output voltage increases as the voltage level of the soft start signal increases.
[0008]
When the output voltage exceeds a certain set voltage level, the output of the error amplifier is inverted, the photocoupler is turned on, and the soft start signal is switched to the generation of the PWM waveform based on the normal feedback level signal.
[0009]
As a result, the output voltage of the switching power supply device is controlled to gradually rise to a predetermined voltage level.
[0010]
[Problems to be solved by the invention]
In the soft start control technology of the switching power supply device described above, when switching from the soft start signal to the feedback level signal, a response delay of an error amplifier, a photocoupler, etc. occurs, resulting in an overshoot of the output voltage. There is a problem that a voltage exceeding the rated voltage is generated.
[0011]
SUMMARY OF THE INVENTION An object of the present invention is to provide a switching power supply device that can reliably prevent an overshoot that occurs when a power supply is started.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, a switching power supply according to the present invention is an insulated switching power supply in which a primary winding side and a secondary winding side are insulated by a transformer and a photocoupler is used for feedback. An auxiliary circuit that operates the photocoupler before the output voltage reaches a predetermined voltage is provided, and the auxiliary circuit is driven by an auxiliary power supply voltage that is higher than the output voltage obtained by rectifying the voltage on the secondary winding side of the transformer. It is characterized by doing.
[0013]
According to such an invention, it is possible to eliminate the overshoot of the output voltage at the time of starting the power supply, and it is possible to supply a stable switching power supply.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described more specifically with reference to the drawings. Here, in the accompanying drawings, the same reference numerals are given to the same members, and duplicate descriptions are omitted. The embodiment of the invention is a particularly useful embodiment in which the present invention is implemented, and the present invention is not limited to the embodiment.
[0015]
1 is a circuit diagram of a switching power supply device according to an embodiment of the present invention, FIG. 2 is a timing chart showing voltage waveforms of respective parts in the switching power supply device of FIG. 1, FIG. 3 is a circuit diagram of a prior art, and FIG. FIG. 5 is a circuit diagram of a switching power supply device according to another embodiment of the present invention, and FIG. 5 is a timing chart showing voltage waveforms at various parts in the switching power supply device of FIG.
[0016]
In this embodiment, the switching power supply device 1 includes a step-down converter (second converter) 2, a half-bridge converter (first converter) 3, a drive control circuit 4, and a feedback control circuit 5.
[0017]
The step-down converter 2 steps down the voltage Vin of the input power supply to a certain voltage level and outputs it. This step-down converter 2 includes a switch element (third switch element) 6, a diode 7, and an inductor 9.
[0018]
The half bridge converter 3 converts the voltage generated by the step-down converter 2 into a DC voltage having a certain voltage level, outputs an output voltage Vo, and supplies the output voltage Vo to the load.
[0019]
The half-bridge converter 3 includes switch elements (first switch element 10 and second switch element 11), capacitors 12 to 14, a transformer 15, diodes 16 and 17, and an inductor 19.
[0020]
The switch elements 6, 10, and 11 are made of transistors such as MOS-FETs, for example. The switch element 6 is controlled to be turned ON / OFF by a drive control signal OUT generated by the drive control circuit 4.
[0021]
The drive control circuit 4 is provided with a comparator 4a and the like. For example, the drive control circuit 4 includes a single package semiconductor integrated circuit device such as an IC (Integrated Circuit), and generates a PWM (Pulse Width Modulation) signal as the drive control signal OUT.
[0022]
Similarly, in the switch elements 10 and 11, the ON / OFF operation is controlled by a control signal (different from OUT) generated by the drive control circuit 4.
[0023]
An input power source Vin is connected to one connection portion of the switch element 6, and the other connection portion of the switch element 6 has one of a cathode of a diode 7 that is a rectifying element for reflux and one of a smoothing inductor 9. Each connection is connected.
[0024]
One connection portion of the capacitor 12 and one connection portion of the switch element 10 are connected to the other connection portion of the inductor 9. One input part on the primary winding side of the transformer 15 and one connection part of the switch element 11 are connected to the other connection part of the switch element 10.
[0025]
In addition, one connection portion of the capacitor 13 and the other input portion on the primary winding side of the transformer 15 are connected to the other connection portion of the capacitor 12. As a result, a half-bridge circuit is configured.
[0026]
A reference potential (GND) is connected to the anode of the diode 7, the other connection portion of the switch element 11, and the other connection portion of the capacitor 13.
[0027]
The control terminal (gate) of the switch element 6 is connected so that the drive control signal OUT output from the drive control circuit 4 is input, and the control terminals (gates) of the switch elements 10 and 11 are similarly connected. The control signals output from the drive control circuit 4 are connected to each other.
[0028]
Further, one connection portion of the capacitor 4b is connected to the negative side (−) input terminal of the comparator 4a, and one connection portion of the capacitor 4c is connected to one positive side (+) input terminal of the comparator 4a. And a reference voltage _VREF . A reference potential (GND) is connected to the other connection portion of the capacitors 4b and 4c.
[0029]
The capacitor 4b is regularly charged and discharged at a certain interval, and generates a sawtooth signal (third comparison signal) CT. The capacitor 4c is charged with a constant current from the reference voltage _VREF, and generates the voltage as a soft start signal (first comparison signal) SS.
[0030]
The cathode of the diode 16 is connected to one terminal on the secondary winding side of the transformer 15, and the cathode of the diode 17 is connected to the other terminal on the secondary winding side of the transformer 15. The anode of the diode 16 is connected to the anode of the diode 17 and the other connection portion of the capacitor 14.
[0031]
One connecting portion of the inductor 19 is connected to the center tap on the secondary winding side of the transformer 15. One connection portion of the capacitor 14 is connected to the other connection portion of the inductor 19.
[0032]
The output voltage Vo of the switching power supply device 1 is output from both connection portions of the capacitor 14.
[0033]
Further, the feedback control circuit 5 includes an auxiliary power supply unit 5a, a photodiode unit 5b, and an auxiliary circuit 5c. The auxiliary power supply unit 5a includes diodes 20 and 21 and a capacitor 22, and generates an auxiliary power supply voltage VH as described later.
[0034]
The photodiode unit 5b includes a photodiode 23a that is a light emitting side of the photocoupler, an error amplifier 24, and resistors 25 to 28, and the auxiliary circuit 5c includes a capacitor 29, a diode 30, and a resistor 32.
[0035]
A reference voltage V _REF is supplied to one connection portion of the resistor 25. A constant current circuit 40 that supplies a constant current from the _VREF terminal is connected between the _VREF terminal and one positive (+) input terminal of the comparator 4a. The other connection portion of the resistor 25 is connected to the collector of the phototransistor 23b, which is the light receiving side of the photocoupler, and to the other positive side (+) input terminal of the comparator 4a. The emitter of the phototransistor 23b is connected to a reference potential (GND).
[0036]
A voltage output from the other connection portion of the resistor 25 is input as a feedback signal (second comparison signal) FB of the drive control circuit 4.
[0037]
Further, the cathodes of the diodes 16 and 17 are connected to the anodes of the diodes 20 and 21, respectively. The cathode voltage of the diodes 20 and 21 becomes the auxiliary power supply voltage VH.
[0038]
The auxiliary power supply voltage VH is supplied to one connection portion of the capacitors 22 and 29, one connection portion of the resistor 26, and the power supply input portion of the error amplifier 24. The other connection portion of the resistor 26 is connected to the anode of the photodiode 23a, and the cathode of the photodiode 23a is connected to the output portion of the error amplifier 24.
[0039]
The resistors 27 and 28 are connected in series between the output voltages Vo. The divided voltages divided by the resistors 27 and 28 are connected so as to be input to the negative (−) input terminal of the error amplifier 24, and the positive (+) input terminal of the error amplifier 24 is connected to The reference voltage V _REF1 is connected.
[0040]
The anode of the diode 30 is connected to the other connection portion of the capacitor 29 and one connection portion of the resistor 32, that is, to the midpoint between the capacitor 29 and the resistor 32. The cathode of the diode 30 is connected to the middle point of the resistor 27 and the resistor 28, and the negative (−) input terminal of the error amplifier 24 is connected.
[0041]
The error amplifier 24 compares the voltage input to the negative (−) input terminal with the reference voltage V _REF1, and when the voltage input to the negative (−) input terminal becomes higher than the reference voltage V _REF1 , the error amplifier 24 is at a low level. It becomes an output and controls the current of the photodiode 23a.
[0042]
The output voltage Vo (−) is connected to the other connection portion of the capacitor 22 and the other connection portion of the resistor 32.
[0043]
Next, the operation at the time of startup of the switching power supply device 1 according to the present embodiment will be described using the signal timing charts of FIG. 1 and FIG.
[0044]
In FIG. 2, from the upper side to the lower side, the feedback signal FB, the signal CT, the soft start signal SS input to the comparator 4a, the drive control signal OUT output from the comparator 4a, and the switching power supply 1 are output. The signal timing of the output voltage Vo is shown respectively.
[0045]
First, when the switching power supply device 1 is activated, a signal CT is output. Thereafter, the capacitor 4c is charged with a constant current by the reference voltage _VREF , and the voltage of the soft start signal SS gradually increases.
[0046]
At this time, since the output voltage Vo is not output, no voltage is generated in the auxiliary power supply voltage VH, the error amplifier 24 does not operate, and the feedback signal FB is also at the reference voltage V _REF level.
[0047]
Therefore, the comparator 4a compares the signal CT with the soft start signal SS, outputs the drive control signal OUT only during a period when the voltage level of the signal CT is lower than the soft start signal SS, and drives the switch element 6. .
[0048]
Since step-down converter 2 is driven by ON / OFF of switch element 6, the voltages of capacitors 12 and 13 (capacitor 12 and capacitor 13 have the same capacity) gradually increase.
[0049]
The switch elements 10 and 11 are alternately turned ON / OFF at a constant duty by a control signal (different from OUT) output from the drive control circuit 4.
[0050]
When the switch element 10 is turned ON, a voltage of ½ of the voltage level stepped down by the step-down converter is applied to the primary winding side of the transformer 15, and this voltage becomes a secondary side voltage proportional to the turns ratio of the transformer 15. The converted secondary current flows to the capacitor 14 via the diode 17 and the inductor 19.
[0051]
Further, when the switch element 10 is turned off and the switch element 11 is turned on, a reverse voltage is applied to the primary winding side of the transformer 15. Therefore, current flows through the capacitor 14 via the diode 16 and the inductor 19 on the secondary winding side of the transformer 15. Along with this, the output voltage Vo gradually increases in voltage level. Similarly, the auxiliary power supply voltage VH gradually increases, and the error amplifier 24 enters an operating state. At this time, the output of the error amplifier 24 is a High output, and the feedback signal FB is continuously at the reference voltage V _REF level.
[0052]
Since the output voltage Vo is applied to the resistor 27 and the resistor 28, a current obtained by the output voltage Vo / (resistance value of the resistor 27 + resistance value of the resistor 28) (hereinafter, this current is referred to as “first current”). ) Flows.
[0053]
Furthermore, since the diodes 20 and 21 are respectively connected to the cathodes of the diodes 16 and 17, that is, both terminals on the secondary winding side of the transformer 15, the auxiliary power supply unit 5 a has about twice the output voltage Vo ( 2Vo) auxiliary power supply voltage VH is generated. The auxiliary power supply voltage VH output from the diodes 20 and 21 is smoothed by the capacitor 22.
[0054]
Then, a current for charging the capacitor 29 from the auxiliary power supply voltage VH flows through a path from the diode 30 to the resistor 28 and a path of the resistor 32 (hereinafter, this current is referred to as “second current”).
[0055]
Here, since the capacitor 29 is not charged at the time of startup, the sum of the first current and the second current flows through the resistor 28. Therefore, the voltage signal Vs having a steeper slope than when only the first current flows through the resistor 28 is input to the negative side (−) input terminal of the error amplifier 24.
[0056]
The error amplifier 24 compares the input voltage signal Vs with the reference voltage _VREF1 . At this time, since the slope of the voltage signal Vs at the time of startup is steep, the voltage signal Vs reaches the reference voltage V _REF1 earlier than when only the first current flows through the resistor 28. When the reference voltage _VREF1 is reached, the output of the error amplifier 24 is inverted to become a Low output, and the photocoupler is driven. As a result, the voltage level of the feedback signal FB starts to fall with a delay of the operation delay time TD between the error amplifier 24 and the photocoupler.
[0057]
On the other hand, since the voltage level of the soft start signal SS increases as the capacitor 4c is charged, when the voltage level of the soft start signal SS becomes higher than the voltage level of the feedback signal FB, the comparator 4a The drive control signal OUT is generated based on the signal FB and the signal CT.
[0058]
As described above, since the first and second currents flow at the start-up, the voltage signal Vs reaches the reference voltage V _REF1 earlier than when only the first current flows, and the output of the error amplifier 24 becomes Low. Since the feedback signal FB can be lowered, the switching power supply device 1 shifts from the soft start control at the time of power activation to the normal control operation before the output voltage Vo becomes an overshoot.
[0059]
When the charging of the capacitor 29 is completed, only the first current flows. However, since the diode 30 is present, the first current does not flow into the resistor 32. Therefore, the auxiliary circuit 5 c including the capacitor 29, the resistor 32, and the diode 30 operates only when the switching power supply device 1 is activated.
[0060]
The operation of the conventional switching power supply 50 will be described.
[0061]
As shown in FIG. 3, the switching power supply device 50 includes an insulated power supply circuit 51, a primary control circuit 52, and a feedback circuit 53. The insulated power supply circuit 51 has a circuit configuration similar to that of the step-down converter 2 and the half-bridge converter 3 shown in FIG.
[0062]
The primary control circuit 52 includes a comparator 54 and capacitors 55 and 56, and performs ON / OFF control of the switch element of the step-down converter. The feedback circuit 53 includes a photodiode 57a, an error amplifier 58, and resistors 59 to 62 on the light emitting unit side of the photocoupler.
[0063]
One connection portion of the capacitor 55 is connected to the negative (−) input terminal of the comparator 54. Further, between the reference one positive side of the voltage _{V REF20} terminal and the comparator 54 (+) input terminal, the constant current circuit 41 for supplying a constant current from the reference voltage _{V REF20} is connected. The other positive side (+) input terminal of the comparator 54 is connected to the reference voltage V _REF20 and one connection portion of the capacitor 56. A reference potential (GND) is connected to the other connection portion of the capacitors 55 and 56, respectively.
[0064]
The capacitor 55 generates a sawtooth signal CT, and the capacitor 56 generates a soft start signal SS. The other positive (+) input terminal of the comparator 54 is connected to the collector of a phototransistor 57b which is the light receiving part side of the photocoupler.
[0065]
The reference voltage V _REF20 is connected to one connection portion of the resistor 59, and the collector of the phototransistor 57b is similarly connected to the other connection portion of the resistor 59.
[0066]
An auxiliary voltage source 70 is supplied to one connection portion of the resistor 60 and the error amplifier 58. The other connection portion of the resistor 60 is connected to the anode of the photodiode 57a, and the cathode of the photodiode 57a is connected to the output terminal of the error amplifier 58.
[0067]
The resistors 61 and 62 are connected in series between the output voltages Vo of the switching power supply device 50, and the negative input terminal of the error amplifier 58 is connected to an intermediate node between the resistors 61 and 62. The reference voltage V _REF10 is connected to the positive input terminal of the error amplifier 58.
[0068]
FIG. 4 is a timing chart of the operation signal of each part in the switching power supply device 50. From the upper side to the lower side, the feedback signal FB, the signal CT, the soft start signal SS, and the output from the comparator 54 are input to the comparator 54. The switching element drive signal and the signal timing of the output voltage Vo output from the switching power supply device 50 are respectively shown.
[0069]
When the switching power supply 50 is activated, a signal CT is output, the capacitor 56 is charged with a constant current by the reference voltage _VREF20 , and the voltage of the soft start signal SS gradually increases.
[0070]
The comparator 54 compares the signal CT with the soft start signal SS, outputs the drive signal OUT only during a period when the voltage level of the signal CT is lower than the soft start signal SS, and turns on the switch element of the step-down converter. As a result, the output voltage Vo gradually increases. Since the voltage obtained by dividing the output voltage Vo by the resistors 61 and 62 and the reference voltage _VREF10 are input to the error amplifier 58 and compared with each other, the output of the error amplifier 58 is output when the output voltage Vo reaches a certain voltage. Is inverted and becomes Low output. Then, a current flows from the auxiliary voltage source 70 to the photodiode 57a, and the phototransistor 57b is turned on.
[0071]
Accordingly, a slight operation delay time TD occurs until a current starts to flow through the photodiode 57a and the phototransistor 57b is turned on in response to this. Then, the feedback signal FB starts to fall from here.
[0072]
As shown in FIG. 4, immediately after the fall of the feedback signal FB, the feedback signal FB has a higher voltage level than the soft start signal SS. Therefore, the voltage level of the feedback signal FB is the drive control signal OUT. Is not reflected in the generation of.
[0073]
For this reason, an overshoot occurs in the output voltage Vo, which may impair the reliability of the switching power supply device 50.
[0074]
On the other hand, according to the switching power supply device 1 shown in FIG. 1, feedback control is performed by the auxiliary power supply voltage VH having a voltage level of about twice the output voltage Vo at the time of soft start control at the time of power activation. The overshoot of the output voltage Vo due to can be prevented, and the reliability of the switching power supply device 1 can be improved.
[0075]
In the present embodiment, the auxiliary power supply voltage VH generated by the auxiliary power supply unit 5a is supplied to the auxiliary circuit 5c. However, when the voltage level of the output voltage Vo is large, as shown in FIG. The voltage supplied to the auxiliary circuit 5c may be taken from the output voltage Vo.
[0076]
In this case, the circuit connection of the switching power supply device 1a is different only in that the capacitor 29 of the auxiliary circuit 5c is connected to one output terminal that outputs the output voltage Vo. For other circuit configurations, FIG. This is the same as the switching power supply apparatus 1 of FIG.
[0077]
In the present embodiment, the switching power supply device is configured by the step-down converter and the half-bridge converter. However, the present invention is not limited to the step-up converter, the buck-boost converter, the push-pull converter, or the full-bridge converter. It can be applied to a switching power supply device having a converter of a type.
[0078]
【The invention's effect】
As is apparent from the above description, the present invention can provide the following effects.
(1) The overshoot of the output voltage due to the feedback delay at the time of starting the power supply of the switching power supply can be surely prevented.
(2) Thereby, the output voltage generated by the switching power supply can be stably supplied, and the reliability of the switching power supply can be improved.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of a switching power supply device according to an embodiment of the present invention.
2 is a timing chart showing voltage waveforms at various parts in the switching power supply device of FIG. 1; FIG.
FIG. 3 is a circuit diagram of a conventional switching power supply device.
4 is a timing chart showing voltage waveforms at various parts in the switching power supply device of FIG. 3;
FIG. 5 is a circuit diagram of a switching power supply according to another embodiment of the present invention.
[Explanation of symbols]
1, 1a Switching power supply 2 Step-down converter (second converter)
3 Half-bridge converter (first converter)
4 Drive control circuit 4a Comparator 4b, 4c Capacitor 5 Feedback control circuit 5a Auxiliary power supply unit 5b Photodiode unit 5c Auxiliary circuit 6 Switch element (third switch element)
7 Diode 9 Inductor 10 Switch element (first switch element)
11 Switch element (second switch element)
12-14 Capacitor 15 Transformer 16, 17 Diode 19 Inductor 20, 21 Diode 22 Capacitor 23a Photodiode 23b Phototransistor 24 Error Amplifier 25-28 Resistor 29 Capacitor 30 Diode 32 Resistor 40 Constant Current Circuit 41 Constant Current Circuit 50 Switching Power Supply Device 51 Isolated power supply circuit 52 Primary control circuit 53 Feedback circuit 54 Comparator 55, 56 Capacitor 57a Photo diode 57b Photo transistor 58 Error amplifier 59 to 62 Resistor 70 Auxiliary voltage source CT signal (third comparison signal)
FB feedback signal (second comparison signal)
OUT drive control signal SS Soft start signal (first comparison signal)
TD operation delay time VH auxiliary supply voltage Vin input supply Vo output voltage _{V REF} reference voltage _{V REF1} reference voltage _{V Ref10} reference voltage _{V REF20} reference voltage Vs voltage signal

Claims (3)

An insulated switching power supply device in which a primary winding side and a secondary winding side are insulated by a transformer and a photocoupler is used for feedback,
An auxiliary circuit for operating the photocoupler before the output voltage reaches a predetermined voltage is provided .
Driving the auxiliary circuit with an auxiliary power supply voltage having a voltage level higher than an output voltage obtained by rectifying the voltage on the secondary winding side of the transformer;
The switching power supply device characterized by the above-mentioned. Compare the reference voltage and the divided voltage by the two resistors, the photodiode on the light emitting side of the photocoupler, two resistors connected in series to the output voltage, and the cathode of the photodiode connected to the output 2. The switching power supply device according to claim 1, further comprising an error amplifier. The auxiliary circuit includes a capacitor and a resistor connected in series to a voltage higher than an output voltage obtained by rectifying the voltage on the secondary winding side of the transformer, and an anode connected to a midpoint between the capacitor and the resistor. 3. The switching power supply device according to claim 2, further comprising a diode having a cathode connected to a midpoint of the two resistors connected to the input of the error amplifier.

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